Infrared 3–4 μm Spectroscopy of Nearby PG QSOs and AGN–Nuclear Starburst Connections in High-Luminosity AGN Populations

Abstract We present the results of infrared L-band (3–4\ m) slit spectroscopy of 30 PG QSOs at z 0. 17, a representative sample of local high-luminosity, optically selected AGNs. The 3. 3\ m polycyclic aromatic hydrocarbon (PAH) emission feature is used to probe nuclear (a few kpc) starburst activity and to investigate the connections between AGNs and nuclear starbursts in PG QSOs. The 3. 3\ m PAH emission is detected in the individual spectra of 5/30 of the observed PG QSOs. We constructed a composite spectrum of PAH-undetected PG QSOs and discerned the presence of the 3. 3\ m PAH emission therein. We estimated the nuclear-starburst and AGN luminosities from the observed 3. 3\ m PAH emission and 3. 35\ m continuum luminosities, respectively, and found that the nuclear-starburst-to-AGN luminosity ratios in PG QSOs are similar to those of previously studied AGN populations with lower luminosities, suggesting that AGN–nuclear starburst connections are valid over a wide luminosity range of AGNs in the local universe. The observed nuclear-starburst-to-AGN luminosity ratios in PG QSOs with available supermassive black-hole masses are comparable to a theoretical prediction based on the assumption that the growth of a supermassive black hole is controlled by starburst-induced turbulence.